Many kinds of electronic circuits such as voltage regulators and analog-to-digital converters compare voltages as a fundamental part of their operation. For example, an operational amplifier (op amp) compares two voltages applied to its input, and then amplifies the detected voltage difference. One of the applied voltages may be an external voltage that varies depending on operating conditions and events, while the other input is a relatively fixed voltage known as a reference voltage.
Ideally, the reference voltage would be a truly fixed voltage that never varied in voltage. However, reference voltages in real-life systems often vary significantly with variations in temperature, power-supply voltage, noise from other circuits, loading of its output, etc. Circuit designers try to minimize these variations in reference voltages by careful and creative circuit design.
While an external reference could be applied to a system, the high level of integration desired in today's systems provides significant cost and size savings when the reference is generated on-chip. Bandgap voltage reference circuits are widely popular for internal reference circuits, since the reference voltage ultimately depends on well-understood semiconductor device properties such as voltages produced at p-n junctions.
Traditional bandgap reference circuits used bipolar transistors such as PNP transistors. However, most high-integration systems use complementary metal-oxide-semiconductor (CMOS) technology. A hybrid technology with both CMOS and bipolar transistors known as BiCMOS has been used, but it is more expensive than standard CMOS. Additional mask layers are often used with BiCMOS, and power consumption is usually higher.
The higher circuit density on today's integrated circuit chips is made possible by shrinking device sizes. These smaller transistors have thinner insulator layers and are not able to withstand voltages that were used just a few years ago. Power supply voltages of 5 volts could break down transistors that are now designed for 2 or 1.5-volt power supplies.
While power supply voltages have been reduced to prevent damage to the smaller transistors used in today's circuits, other fundamental device characteristics have not tracked. For example, the transistor threshold voltage has remained at about 0.7 to 0.5 volts. As further device shrinks require that power supplies be reduced to near 1.0 volt, design of circuits that can still operate and turn on transistors using a 0.5 to 0.7 volt threshold is challenging.
Many bandgap reference circuits can only operate using power supply voltage above 2 volts. Some bandgap reference circuits that are designed to operate with 1-volt supplies suffer from low current amplification (low beta), and sacrifice current drive strength to achieve low-voltage operation. Poor power-supply rejection ratios (PSRR) and noise due to large impedances are typical.
Bandgap reference circuits can be difficult to implement when the power supply is close to 1 volt since turning on an op amp from a PNP transistor reference requires that the NMOS transistor threshold voltage Vth be less than the base-emitter junction voltage Vbe. Since Vtn and Vbe are close to each other, process yields may be low due to this requirement.
Bandgap reference circuits may feed into amplifiers that will amplify any noise that is injected into the sensitive bandgap reference circuit. Noise may be fed back into the bandgap reference circuit from its load, especially when the load is insufficiently driven by a low current drive amplifier. The reference voltage may then fluctuate due to loading noise, and this noise may even be amplified.
What is desired is a bandgap reference circuit that can be implemented in a standard CMOS process using a parasitic PNP transistor. A bandgap reference circuit that can operate with a 1-volt power supply and yet still have a high current drive to its load is desirable. A bandgap reference circuit with a low startup voltage and good line regulation and noise rejection is desirable. A bandgap reference circuit that compensates for temperature is desirable.